1. Field of the Invention
The present invention relates to a method of determining a measuring time for an analog-digital converter and, more particularly, to a method of determining a measuring time for an analog-digital converter used for measuring minute signals with high accuracy.
2. Description of the Related Art
Conventionally, an apparatus called a source monitor unit (SMU) has been used to test the characteristics of semiconductor devices, such as an FET or IC. A semiconductor testing apparatus usually has a plurality of SMUs, each of which is connected to each terminal of a semiconductor device to be measured. A main measuring circuit comprising an SMU controller, a microprocessor, a RAM, and a ROM is connected to each SMU via an analog-digital converter and a digital-analog converter for input and output, respectively. The analog-digital converter is used to digitize signals from a semiconductor device to be measured. The digital-analog converter is used to convert digital signals from the main measuring circuit into analog signals and supply current and voltage to the semiconductor device to be measured. In such cases, various analog-digital converters are used for different purposes according to their characteristics. A semiconductor testing apparatus is usually equipped with a plurality of types of analog-digital converters, such as integral analog-digital converters and successive comparison analog-digital converters.
In recent years the operating voltage and power consumption of a semiconductor circuit have decreased considerably, so smaller current or voltage values need to be measured. The measuring range of an analog-digital converter therefore has widened. The higher its resolution becomes, the stricter the signal-noise ratio requirement becomes.
Spot measurements, being the most basic measurements for a semiconductor testing apparatus using an SMU, will be performed in compliance with the following procedure:
1. Measure the current running through the SMU and search for a current range with which the highest resolution can be obtained;
2. Change the current range of the SMU to the suitable current range found;
3. Connect the input to an analog-digital converter and the output from the SMU;
4. In order to obtain the desired signal-noise ratio, perform analog-digital conversion after setting integrating time or the number of times measured values are averaged, depending on an analog-digital converter used; and
5. Treat digital data thus obtained as a result of this measurement.
In this case, noise must be eliminated from signals to be measured in order to obtain the desired signal-noise ratio for an analog-digital converter. Noise to be considered here are usually classified into three categories: (1) 1/f noise, (2) white noise (also called thermal noise), and (3) a noise component synchronizing with the power supply frequency. It is difficult to eliminate 1/f noise. Noise synchronizing with power supply voltage can be largely eliminated by setting the measuring time longer than or equal to one cycle of the power supply. White noise can be reduced by setting the measuring time represented by integrating time or the number of times of measurements over which measured values are averaged (the number of times of measurements over which measured values are averaged is the number of times measurements are performed with a successive comparison analog-digital converter, and an increase in this number represents a longer measuring time).
In principle, the longer the measuring time for analog-digital conversion becomes, the more noise can be eliminated. However, it takes a longer time to measure. Main factors which determine the magnitude of noise components included in the signal to be measured are (1) set voltage range, (2) current range used for measurement, and (3) strength of current running through an SMU. In order to obtain the desired signal-noise ratio, it is necessary to determine the measuring time for analog-digital conversion with the above factors taken into consideration and perform analog-digital conversion. A conventional technique has taken into consideration only (1) the set voltage range and (2) the current range used for measurement described above and has determined the measuring time for analog-digital conversion statically on the basis of a table (or relational expression). The measuring time of analog-digital conversion for very small signals is shown in Table 1. In this example, this table is applicable to any of set voltage ranges of 2 V, 20 V, 40 V, and 100 V, and any value within any of these ranges can be used as the set voltage. The measuring time of analog-digital conversion is 16 PLCs (power line cycles) and is uniform. (1 PLC is equal to 20 milliseconds with power supply of 50 cycles/second.) The strength of current running through an SMU described above is not taken into consideration, so this value is determined on the assumption that the signal-noise ratio is worst (when the value of the current running through an SMU is smaller than or equal to 10% of the maximum measurable current value). Therefore, if a current value measured is large compared to the maximum current value, then the efficiency of the measuring time is low. Making it possible to measure even smaller currents will strengthen this tendency, resulting in more wasteful use of the measuring time.
FIG. 1 is a graph showing the relations between the signal-noise ratios for thermal noise and the measured current values for measuring times of 1, 2, 4, 8, and 16 PLCs. This graph shows theoretical values at 23xc2x0 C. and in the 10 pA range (range resistance 45 Gxcexa9).
Another conventional technique uses two-dimensional tables for the measuring time with the output current and output voltage ranges of an SMU taken into consideration for the purpose of reducing noise. Such tables are held in a semiconductor testing apparatus as internal data. These tables are prepared according to the desired resolution or signal-noise ratio range and one of them is selected for actual use. This method makes it possible to select the best table if a signal to be measured is within a limited range. However, if a signal to be measured changes significantly during the sweep measurement, shortening the analog-digital conversion time for the purpose of shortening the measuring time will lower the signal-noise ratio for very small signals. Moreover, increasing the signal-noise ratio by lengthening the analog-digital conversion time will lead to the use of longer measuring times for large signals, which will lengthen the entire measuring time. The compatibility of high resolution with the quickness of measurements therefore has been desired.
An object of the present invention therefore is to shorten the measuring time for an analog-digital converter without affecting the resolution.
The present invention provides a method for determining the measuring time for an analog-digital converter in which the measuring time for an analog-digital converter is determined according to voltage and current ranges selected for a current to be measured and a current value measured preliminarily.
Furthermore, the present invention provides a method of determining measuring time for an analog-digital converter which comprises the steps of preliminarily measuring a current to be measured, determining a voltage range and a current range used for measurement, and determining the measuring time of an analog-digital converter for current measurement on the basis of the determined voltage and current ranges and the measured current value. In this case, it is preferable that the ratio of the measured current value to a full scale in each current range should be calculated and that measuring time should be determined on the basis of the ratio.
In addition, the present invention provides a method of determining the measuring time for analog-digital conversion which comprises the steps of generating a plurality of functions defining measuring time for different analog-digital converters as functions of a voltage range and current range for current to be measured, selecting two or more from the plurality of functions generated, and combining the selected functions, and selecting a comparatively long measuring time for a comparatively weak-current signal and a comparatively short measuring time for a comparatively strong-current signal from measuring time calculated from the selected functions. It is preferable that a boundary range should be set and that the two or more functions should be combined with the boundary range as a boundary. In this case, a function is the fixed relation between two or more variables and may, for example, be mathematical expression or a numerical table.
In this case, it is preferable that an analog-digital converter should be connected to an SMU in a semiconductor testing apparatus. The meaning of measuring time depends on the type of an analog-digital converter. That is to say, it means the integrating time for an integral analog-digital converter and the number of times of measurements over which measured values are averaged for a successive comparison analog-digital converter. The measuring time is also a parameter for analog-digital converters of other types, which corresponds to the integrating time or the number of times measured values are averaged.